As an emission type electronic displaying device, there is an electroluminescence display (hereinafter referred to as ELD). As element constituting the ELD, there are mentioned an inorganic electroluminescence element and an organic electroluminescence element. The inorganic electroluminescence element has been used for a plane-shaped light source, but a high voltage alternating current has been required to drive the element. An organic electroluminescence element (hereinafter also referred to as organic EL element) has a structure in which a light emission layer containing a light emission compound (organic compound thin layer containing fluorescent organic compound) is sandwiched between a cathode and an anode, and an electron and a hole were injected into the light emission layer and recombined to form an exciton. The element emits light, utilizing light (fluorescent light or phosphorescent light) generated by inactivation of the exciton. To utilize the emitted light, at least one of the electrodes each arranged at both sides of the organic compound thin layer is usually a transparent electrode such as ITO. The transparent electrode is supported by a transparent substrate such as a glass plate.
The organic electroluminescence element is noted from the viewpoint of the portability and space saving since the element can emit light at a low voltage within the range of from several to several decade volts and has a wide viewing angle and a high visuality since the element emits light itself and is complete solid thin layer shaped element.
However, development of an organic electroluminescence element for practical use is required which efficiently emits light with high luminance at a lower power.
The organic electroluminescence element has a problem of low output coefficient of light (a ratio of energy coming out of a substrate to emitting energy) for improving performances thereof. Namely, the light is largely lost when the light is conducted from the light emission layer to the light conversion member since the light emitted from the light-emitting layer has no directivity and scatters in all directions. Accordingly, the intensity of the light is made insufficient and the displayed image becomes too dark.
The light from the light emission layer only emitted to the front direction is utilized. Since the output coefficient of light to the front direction (the light emission coefficient) can be approximated by ½n2 according to the classical optics, the output coefficient of light to the front direction is defined by the refractive index n of the light emission layer. The light emitting coefficient of the organic electroluminescence member is about 20%, when the refractive index of the light emission layer is 1.7. The light other than the above is transported in the area direction of the light emission layer (the scatter in the side direction), or disappeared in the metal electrode facing to the transparent electrode through the light emission layer (an absorption to the rear direction). That is to say that, in the organic electroluminescence element, light is emitted in a layer whose refractive index (the refractive index is about 1.7 to 2.1) is higher than that of air, and only 15 to 20% of the light emitted in the light emission layer can be extracted. This is because light which enters a boundary (a boundary between a transparent substrate and the atmosphere) at an angle θ larger than a critical angle is totally reflected and cannot be extracted from the element, or because light is totally reflected at a boundary between the transparent substrate and the transparent electrode or between the transparent substrate and the light emission layer, so that the light exits from the side of the element through the transparent electrode or the light emission layer.
Several methods were investigated for improving this output coefficient of light. For example, as methods to improve the light extraction efficiency, there are a method to form concavity and convexity on the surface of the transparent substrate to prevent total internal reflection at a boundary between the transparent substrate and atmospheric air (referred to Patent Document 1); a method to form a flat layer having an intermediate refractive index between the substrate and the light emission layer to form an anti-reflection layer (referred to Patent Document 2); a method to form a flat layer having a lower refractive index than a substrate glass between the substrate and the light emission layer (referred to Patent Document 3); and a method to form a diffraction lattice at a boundary between any two of the substrate, the transparent electrode and the light emission layer (including a boundary between the substrate and atmospheric air) (referred to Patent Document 4).
In a method of forming concavity and convexity on a surface of a transparent substrate or a diffraction grating, the asperity is generally formed by etching using photolithographic technique, however, it results in low productivity and high cost. Further, in a method of forming a flat layer having intermediate refractive index or a method of introducing a flat layer having lower refractive index than a substrate glass between a substrate glass and a light-emitting member, a boundary having different refractive indexes exists after all, resulting in poor improvement in an output coefficient of light.
As simpler method than above, investigated was a method in which a film having a gradient refractive index structure between a transparent substrate and a transparent electrode was formed (referred to Patent Document 5). However, in this method, since a ratio of two components is needed to be changed continuously so as to forming a gradient refractive index structure, it causes non-uniformity in a film stress and may result in a fatal defect in a film property such as crack or peeling of film.
According to the present invention, on of features is to employ a transparent resin film as a substrate. In order to maintain a flexibility of the transparent substrate as well as an abrasion resistance, a film adhesion and anti-curling, required is to employ hard coats on both surfaces. Therefore, it was found to be important to balance refractive indexes or thickness between these 4 layers as the transparent resin film, two hard coat layers, and the transparent electrode layer.